With in-depth development of semiconductor technology, conventional light sources are gradually replaced with semiconductor light-emitting devices such as LEDs or semiconductor lasers due to their advantages such as high efficiency, long life, difficult to be dilapidated, and high reliability, and the semiconductor light-emitting devices have been used widely.
Generally, the semiconductor light-emitting device is driven in a constant-current manner. Taking the LED as an example, FIG. 1 is a schematic diagram illustrating that the LED is driven in a common constant-current manner. In FIG. 1, a LED driver 1 is used to drive a LED module 2, and a constant-current control unit 3 samples a current average value of the LED module 2 from a sampling point A, then feeds back the sampled current average value to the LED driver 1, and then the LED driver 1 adjusts the LED module 2 based on the current average value. The LED module 2 (i.e., a load) in FIG. 1 may be a single LED light, a LED light string composed of a plurality of LEDs connected in series, or LED lights composed of a plurality of LED strings connected in series or in parallel. Moreover, the LED module 2 may include other circuits such as a current balancing circuit, a filter capacitor or a protection circuit.
The existing LED driving circuit generally relates to applications of a plurality of strings of LED lights. For such LED circuit with multiple strings of LEDs, if a certain string or a plurality of strings of LEDs fail, it is generally required that the remained strings of LEDs can continue operating. Therefore, a protection circuit is generally connected in parallel with the LED string to short out the failed LED string, so as to ensure the remained strings to operate normally. Such protection circuit is, for example, shown in portion (b) of FIG. 2. In the portion (b) of FIG. 2, the protection circuit 21 includes a Zener diode D11, a resistor R11 and a thyristor Q11. The Zener diode D11 is connected in series with the resistor R11; an anode of the Zener diode D11 is connected with a second end of the resistor R11; a gate of the thyristor Q11 is connected to a connection point between the anode of the Zener diode D11 and the second end of the resistor R11; an anode of the thyristor Q11 is connected to a cathode of the Zener diode D11; and a cathode of the thyristor Q11 is connected to a first end of the resistor R11.
Additionally, portion (a) of FIG. 2 is a detailed circuit diagram showing that the LED module 2 having a plurality of strings of LED lights is driven by the constant-current control unit 3. In the portion (a) of FIG. 2, the LED driver 1 includes switching elements S1 and S2, a resonant circuit and a transformer Tr. The resonant circuit includes a resonant inductor Ls and a resonant capacitor Cs connected in series. One end of the resonant circuit is connected to a connection point between the switching elements S1 and S2, and the other end of the resonant circuit is connected to a primary side of the transformer Tr. In addition, the LED module 2 includes a current balancing circuit. The current balancing circuit includes current balancing capacitors C1-C5, rectifier diodes D1-D6 and six groups of LED loads LED1-LED6. Each group of LED load includes a filter capacitor Co1-Co6, a protection circuit 21 and a LED string. The LED string may include one or more LEDs. The switching elements S1 and S2 are connected in series to form a half-bridge switching circuit, so as to convert a DC input voltage into a DC square wave signal and then transfer the DC square wave signal to the resonant circuit and the transformer Tr. The output from a secondary side of the transformer Tr is an AC current source, so as to power the LED module 2 shown on the right side of the portion (a) of FIG. 2. The constant-current control unit 3 is connected between the LED driver 1 and the LED module 2, and may sample the current average value from any sampling point at which the LED current can be reflected. Such sampling point may be, for example, sampling point SA, SB, SC, SD, SE, SF or SG shown in the (a) portion of FIG. 2. The constant-current control unit 3 controls an output of the LED driver 1 according to the sampled current average values. That is, when the LED operates in the constant-current mode, a feedback may be realized by detecting an average value of secondary side currents of the transformer Tr.
However, when a circuit of any string of LEDs fails, the protection circuit 21 shorts out this string of LEDs, and the load of the circuit will change suddenly. Accordingly, a gain of the resonant circuit will change suddenly, and a current which is much larger than a current in a normal state, i.e., an inrush current, will be occurred at the secondary side of the transformer Tr. Since a speed of a feedback loop in the above constant-current mode is not fast enough to perform adjustments to the inrush current in time, a life of the LED is reduced by the inrush current.
According to a conventional manner for avoiding the inrush current, a positive temperature coefficient (PTC) element as shown in FIG. 3 is connected in series to the LED load. Such manner has a low cost, but the introduction of the PTC element may cause an increase of the line impedance so that the loss of line in the normal state increases.
Accordingly, it is very urgent to develop a circuit for suppressing current impact, reducing the current impact and having a low cost, so as to overcome at least in part the above deficiencies in the related art.